Fix this quarter's new clippy issue

[prozacchiwawa]

Note

Low Risk
Mechanical lint-driven refactor with equivalent control flow; no functional or security impact expected.

Overview
Updates OpCons::invoke in serialize.rs to satisfy a new Clippy lint by replacing nested pop().and_then(...) with popping the right item first, then matching on val_stack.pop().zip(item_b).

Behavior is unchanged: build a cons from two stack values when both pops succeed, otherwise return None.

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